The present invention relates to a biodegradable polyester foil comprising:                i) from 80 to 95% by weight, based on the total weight of components i to ii, of a biodegradable polyester based on aliphatic and aromatic dicarboxylic acids and on an aliphatic dihydroxy compound;        ii) from 5 to 20% by weight, based on the total weight of components i to ii, of polyhydroxyalkanoate;        iii) from 10 to 25% by weight, based on the total weight of components i to vi, of calcium carbonate;        iv) from 3 to 15% by weight, based on the total weight of components i to vi, of talc;        v) from 0 to 30% by weight, based on the total weight of components i to vi, of polylactic acid and/or starch;        vi) from 0 to 2% by weight, based on the total weight of components i to vi, of 2-(4,6-bisbiphenyl-4-yl-1,3,5-triazin-2-yl)-5-(2-ethyl-(n)-hexyloxy)phenol.        
The invention further relates to the use of these polyester foils, in particular for agricultural applications, such as mulch foils.
Polyester foils (chill-roll foils of thickness 250 micrometers) made of components i and ii have been mentioned in EP-A 2 330 157. Specifically, mixtures in the range from 25 to 75% by weight of aliphatic-aromatic polyesters (component i) and from 25 to 75% by weight of polyhydroxyalkanoate (component ii), preferably poly(3-hydroxybutyrate-co-3-hydroxyhexanoate), are disclosed. However, those foils are not entirely satisfactory in particular in respect of tear-propagation properties.
It was an object of the present invention to provide biodegradable polyester foils with improved tear-propagation properties.
Surprisingly, it has been found that the polyester foils mentioned in the introduction, with only from 5 to 20% by weight polyhydroxyalkanoate content, have better tear-propagation properties.
It has also been found that still further improvement in tear-propagation properties can be achieved if from 10 to 25% by weight, based on the total weight of components i to v, of calcium carbonate (component iii) is added to the foils of the invention.
WO2002/016468 discloses filled biodegradable polyester foils. WO2010/118041 discloses that addition of high-purity calcium carbonate increases the processing latitude of polyhydroxyalkanoates. WO2010/118041 gives no indication of the effect of the calcium carbonate on the tear-propagation properties of foils made of polyhydroxyalkanoates, and in particular of foils based on the mixture of the invention made of aliphatic-aromatic polyesters and of polyhydroxyalkanoates.
Another object of the present invention was to develop polyester mixtures which have good processability when processed to give blown films. Blowing is usually used to produce thin foils, i.e. foils of thickness smaller than 50 micrometers or preferably smaller than 30 micrometers. This process requires polymer mixtures which exhibit a certain viscosity and bubble stability.